Metal film resistor



March 26, 1957 G. J, KAHAN 2,786,925

METAL FILM RESISTOR Filed Dec. 51, 1952 TE RM/NAL LEAD CONNE C T/ON TERM/NAL LEAD CONNECT/0N THERMAL o/Frus/o'lv BARR/ER RESISTANCE FILM f 16 IN VEN TOR.

GEORGE J. KAHAN. BY

METAL FILM Rusisron George` J. Kahan, Williamstown, Mass., assignor to Sprague Electric Company, North Adams, Mass., 'a corporation of Massachusetts The present invention relates to new and improvedmetallic fihn resistance components.

For `some time it has been known to create resistance units having this metallic film by depositing a metal organic compound or a metal salt composition upon an inert base, and subsequently firing -the so-coated base, for predetermined'perio'ds. l

Satisfactory resistance coatings, perse, can be obtained by the broad process described, but difficulty has arisen in maintaining predetermined characteristics in the resistance filmwhen such is covered with a high temperature vitreous enamel protective surface coating. Such vitreous enamel compositions are normally applied from an aqueous suspension, and are fired at a temperature of about 1000 to 1300 F. for a period of l to 5 minutes. During the firing the metallic resistors undergo changes in resistance values, and may become unsatisfactory with a complete absence of any stability in the operational characteristics. Repeated investigations into the nature of this trouble indicates that it results primarily because the ends of the resistance films adjacent to the terminal elements break down prematurely producing undesirable and uncontrollable discontinuities in the resistance. This is particularly severe when silver is used as terminal lead connections.

One object of the present invention is to prevent instability and break down in vitreous enamel coated resistors employing a metallic resistance layer. Another object of the invention is to provide simple, satisfactory processes for manufacturing metallic film type resistors. Other objects of the invention, as well as the advantages, will become apparent from the specification and claims which follow. In the accompanying drawing the single figure is a sectional view of a resistor pursuant to the present invention.

The generic aspect of the invention may be broadly defined as comprising a process for forming metallic resistance films on non-conductive cores, including the step of associating with such resistance film an element, compound, or composition having at least one constituent of low thermal mobility at or adjacent toits terminal connections. In accordance with the present invention, the above disadvantage is substantially prevented by placing a narrow ring of a second material of low diffusion constant adjacent to the terminal elements prior to firing so as to counteract the diffusion tendency of the first terminal metal.

This feature of the invention is perhaps best illustrated with reference to a specific example. Quarter inch diam eter cylinders of steatite, coated with a 40% silver and 60% palladium resistance layer, were in accordance with one form of the invention provided with a narrow palladium ring overlay adjacent to its ends at about the approximate locations of the silver deposit applied over it in connecting the resistance film with the terminal lead elements, and the unit fired at 700 F. for fifteen minutes. Thereafter, the resistor was coated with a vitreous enamel of the type mentioned above and again fired 2 ata temperature of 1200c F. for about 2 minutes. During the subsequent firing, there was no substantial migration or diffusion of any major portion of the end silver deposits into the resistance layer. It was particularly noted that the resistance film, after the initial firing, did

. not have silver rich alloy areas near the terminal connections as had the units prepared by prior known processes.

The above construction is illustrated in the ligure where the steatite base is shown at 10, the silver palladium alloy coating at 1,2, the thermal diffusion barrier at 14, the the silver terminal lead sites at 16, the leads themselves at 18, and the enveloping vitreous enamel coating at 20.

Although the precise mechanism involved is not completely understood, it is believed the enriching of the alloy deposit near the terminal boundaries with silver permits an alloy of low melting point and high internal mobility to be formed during the firing. The amount of palladium applied did not appear to be critical as long as the thickness of palladium was about ten times thicker Y, than the thickness of the resistance layer. In any event, no diiculty was experienced with excessive diffusion of the silver into either the resistance layer or the ceramic body, andthe resistance film so formed can be satisfactorily coated with common commercial vitreous enamel com-' positions in a conventional manner.

Alternatively, the terminal rings may constitute addi tional bands of the original alloy, although the resistor .so formed is slightly more susceptible to oxidation in the terminal region.

One specific process in which metallic film resistors having exceptional predeterminable characteristics were produced was formulated as follows: A metal organic resinate solution was diluted with acetophenone until it contained about .66% metal. The solution :containing .66% metal was then applied by brush or spraying to a ceramic base. The dried film was then partially decomposed at about 400 F. In this fashion re-dissolution of the metal organosol by the second coat was avoided and independent multiple coats were applied. ln this manner local variations in the thickness of the metal film were also avoided as much as possible. After sufiicient coats to give the desired resistance were applied, the organosol was burned off at 750 F. Good, distinct metallic films having exceptioned stability and low temperature char acteristics were thus obtained.

To each end of the steatite core is applied a terminal. This terminal consists of silver flake bonded to the steatite by material such as powdered glass. It is to be pointed out ythat it is preferable that the terminals are immediately adjacent to the resistance film but do not actually contact said film.

The critical feature of the inventive process is the application of the metallic bands which physically and electri'cally connect the resistance film to the terminals. For a band material of low thermal mobility palladium is preferred with platinum and other noble high melting point metals also being satisfactory.

This band is formed by painting a 5% solution of organic palladium compound of resinous character and thereafter firing at 800 F. for l5 minutes. This cycle was repeated until 'a band having a thickness of about ten times the resistance film was formed.

The resistance unit was then sprayed with a suspension of a commercial vitreous enamel and fired at a temperature of 1000 F. for 3 minutes.

Although the above examples describe silver-palladium resistors only, it will be realized by those skilled in the art that the broad teachings apply to other metallic resistance materials, as for example, gold-palladium, goldplatinum, and silver-platinum alloys. The teachings also apply where the resistance layer consists solely of one patented Mar. 26, 1957 3 element and where theterminal contact is attached by means of a second, more'mobile metal. It will further be apparent that other type core materials may be used, as for example, quartz, boro-silicate glass, lead glass, andthe like.

Also, the terminal rings for prevention of diffusion, migration and oxidation of the constituent having the greater thermal mobility may be applied to tlieA core' either before or after the application of the main resistance lm and terminal contact, or alternatively, between the application of the main resistance lm and the terminals.

As many apparently widely different embodiments of my invention may be made without departing from the spirit and scope hereof, it is to' be understood that my' invention is not 'limited to the specific' embodiments' hereof except as defined in the appended claims.

What is claimed is:

l. An electrical resistance comprising a vitreous base, a metallic alloy resistance film on said base, electrical terminals attached to spaced portions of said film, a narrow band of a metal which counteracts thermal diffusion interposed between said terminals and the portions of the'film to which they are attached and a vitreous coating over the metal portions.

2. A metal film resistor comprising. a vitreous base, an electrically resistive film of a metal alloy upon said base, terminals connected to said base by means of redon silver layers, narrow bands of a metal selectedfrom the class consisting of platinum and palladium connecting said silver layers with the film so as to counteract any tendencies of said silver layersl to' diffuse into s'aid resistance iilm and a vitreous coating over the metal portions.

3. An electrical resistor comprising a vitreous base, an electrically resistive silver alloy film on said base, thermal diffusion barrier coatings of a metal selected from the class consisting `of platinum and palladium on spaced portions of said film, silver terminal connection layers on said barrier coatings, and a vitreous layer covcring the met'al portions. v

4. The invention of claim 3 in'which the barrier coatings are at least about ten times as thick as the resistive film.

5. A metal iilr'n resistor comprising a vitreous base, an electrically resistive film of a metal alloy upon said base, terminals connected to said base by means of firedon silver layers, and narrow bands of a metal selected from the class' consisting of platinum and palladium connecting said silver layers with the iilm so as to counteractrany tendencies of said silver layers to diffuse into said resistance film.

References cited in the sie of this" patent UNITED STATES PATENTS 

5. A METAL FILM RESISTOR COMPRISING A VITREOUS BASE, AN ELECTRICALLY RESISTIVE FILM OF A METAL ALLOY UPON SAID BASE, TERMINALS CONNECTED TO SAID BASE BY MEANS OF FIREDON SILVER LAYERS, AND NARROW BANDS OF A METAL SELECTED FROM THE CLASS CONSISTING OF PLATINUM AND PALLADIUM CONNECTING SAID SILVER LAYERS WITH THE FILM SO AS TO COUNTERACT ANY TENDENCIES OF SAID SILVER LAYERS TO DIFFUSE INTO SAID RESISTANCE FILM. 